monitoring system employing carrier recovery

ABSTRACT

The present disclosure provides a system, apparatus and method to provide for monitoring of characteristics of optical signals, as part of wavelength division multiplexed signals for example, transmitted over a network infrastructure. The characteristics of each optical signal may be monitored and maintained at desired values in order to optimize system performance. A system including a coherent detector, as part of a coherent receiver for example, may be employed to associate each transmitted optical signal with a modulated source. Control signals generated by the system can then be provided to elements of the modulated source to control characteristics, such as optical power, optical frequency, and optical phase, for example, of the transmitted optical signal.

BACKGROUND

Wavelength division multiplexed (WDM) optical communication systems areknown to include one or more photonic integrated circuits (PICs) inwhich multiple optical signals, each having a different wavelength orassociated carrier frequency, are combined into a modulated outputsignal for transmission over an optical fiber. Such systems typicallyinclude transmitters having a laser supplying light at a carrierfrequency, a modulator configured to modulate the light output of thelaser, and an optical combiner to combine each of the modulated outputsinto the modulated output signal. The carrier frequencies associatedwith each of the modulated outputs, collectively the frequenciesdefining a carrier frequency grid, or simply frequency grid, may bespectrally spaced from each other to define a channel spacing withrespect to the grid. The WDM optical communication system may includemultiple PIC devices, the outputs of each being spectrally combined orinterleaved to form a combined output signal for transmission over anoptical fiber. The frequency grids of each of such PIC devices may bespectrally spaced or offset from each other such that a portion of eachfrequency grid overlaps a portion of the remaining frequency grids.Thus, each adjacent signal channel may be from different wavelengthgrids, the adjacent signal channels defining a channel spacing withrespect to the interleaved signal.

Historically, such modulated output signals were amplitude or intensitymodulated. More recently, however, more advanced transmission systems,such as coherent systems, use more complex modulation formats. Suchcomplex modulation formats may employ phase-shift keying (PSK), forexample, which offer higher capacity than the intensity modulatedsignals. Some examples of modulation formats which incorporate PSKinclude binary phase shift keying (“BPSK”), quadrature phase shiftkeying (“QPSK”), differential phase-shift keying (“DPSK”), andpolarization multiplexed differential phase-shift keying (PM-DPSK), toname a few.

As with other transmission systems, coherent transmission systems mayprovide for a data capacity over a corresponding link of a givendistance or reach, the capacity provided within a margin, such as a biterror rate for example. However, links employed for transmission ofoptical signals may include impairments which may limit the performanceof the transmission system. Such impairments may include, for example,various forms of dispersion due to random imperfections and asymmetriesof the optical fiber associated with the link, such as polarization modedispersion or cross phase modulation chromatic dispersion. For purposesherein, impairments shall include any linear or non-linear impairmentwhich may impact the integrity of a transmitted optical signal. Suchimpairments may be associated with the optical signal itself, such asoptical power or a modulation scheme employed to modulate the opticalsignal, or may be associated with structure of the transmission system,such as an optical fiber over which the optical signal is transmitted.

What is needed is a system which monitors characteristics of opticalsignals, as part of a wavelength division multiplexed signal forexample, in order to optimize the optical signals in light ofimpairments which may be present.

SUMMARY

The present disclosure provides a system, apparatus and method todynamically provide variable channel spacing associated with multiplexedor combined signals transmitted over a network infrastructure,wavelength division multiplexed signals for example. In a first aspectof the embodiments of this disclosure, an apparatus is provided whichincludes first, second, and third circuits, the first circuit includingan oscillator and a filter. The oscillator may supply light having afrequency which sweeps at a sweep rate from a first one of a pluralityof frequencies to a second one of the plurality of frequencies. A firstportion of the light may be provided to the filter which filters thefirst portion of the light to provide a filtered output having anintensity peak at a third one of the plurality of frequencies. Thesecond circuit may be configured to receive a second portion of thelight from the first circuit on a first input and an optical signal on asecond input. The optical signal may have a fourth one of the pluralityof frequencies, the second circuit configured to provide an outputsignal in response to the second portion of the light and the opticalsignal. The output signal is indicative of the fourth one of theplurality of frequencies. The third circuit may be configured to providea control signal based on, at least, the filtered output of the firstcircuit, the output signal of the second circuit, and the sweep rate,where the control signal is configured to control a corresponding one ofa plurality of characteristics associated with said optical signal.

In certain embodiments, the filter includes an etalon configured toreceive the first portion of the light and provide the filtered outputas a filtered optical output including the intensity peak at the thirdof the plurality of frequencies. The first circuit may also include aphotodetector configured to receive the filtered optical output andconvert the filtered output into a filtered electrical output, thefiltered electrical output being provided by the first circuit as thefiltered output. The first circuit may also include an optical couplerhaving an input configured to receive the light and provide the firstportion of the light at a first output of the optical coupler and asecond portion of the light at a second output of the optical coupler.

In other embodiments, the oscillator is a laser and the light includes afirst light supplied by a first end of the laser and a second lightsupplied from a second end of the laser, the first portion of the lightbeing the first light. In certain embodiments, the oscillator may be adistributed feedback laser or a distributed Bragg reflector laser. Inyet other embodiments the second circuit is a coherent detector, whichmay include a 90-degree optical hybrid circuit configured to receive theoptical signal and the second portion of the light for processingthereof.

In still other embodiments, the plurality of characteristics areselected from a group including an optical power of the optical signal,a phase of the optical signal, a carrier frequency of the opticalsignal, a carrier frequency of an output of the laser, a wavelength ofthe optical signal, a wavelength of the output of the laser, the gain ofthe amplitude varying element, a bias of the laser, and a bias of themodulator. In yet other embodiments, the apparatus includes a thermaldevice configured to adjust a temperature of the laser, the temperatureof the laser being set in response to the control signal. The thermaldevice may be a heater. The thermal device may be a thermo-electriccooler.

In some embodiments, the intensity peak of the filtered output is afirst of a plurality of intensity peaks of the filtered output, each ofthe plurality of intensity peaks being associated with a respective oneof the plurality of frequencies. Each intensity peak may have anamplitude which is different from the other ones of the plurality ofintensity peaks.

In another aspect of the embodiments of this disclosure, an apparatus isprovided which includes first, second, and third circuits. The firstcircuit may include an oscillator and a filter, the oscillator supplyinglight having a frequency which sweeps at a sweep rate from a first of aplurality of frequencies to a second of the plurality of frequencies. Afirst portion of the light may be provided to the filter which filtersthe first portion of the light to provide a filtered output having anintensity peak at a third of the plurality of frequencies. The secondcircuit may be configured to receive a second portion of the light fromthe first circuit on a first input and a wavelength division multiplexedoptical signal on a second input. The wavelength division multiplexedoptical signal may include a plurality of optical signals, eachincluding a corresponding one of a group of the plurality offrequencies. In response to the second portion of the light and thegroup of the plurality of frequencies, the second circuit may provide anoutput signal, the output signal indicative of the group of theplurality of frequencies of the optical signal. The third circuit may beconfigured to provide a plurality of control signals in response to thefiltered output of the first circuit, the output signal of the secondcircuit, and the sweep rate. Each of the plurality of control signalsmay be configured to control a corresponding one of a plurality ofcharacteristics associated with a corresponding one of the plurality ofoptical signals.

Other objects, features and advantages of the various embodiments of thedisclosure will be apparent from the drawings, and from the detaileddescription that follows below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate one or more implementationsdescribed herein and, together with the description, explain theseimplementations. These drawings are intended to be illustrative, notlimiting. In the drawings wherein like reference symbols refer to likeparts:

FIG. 1A is a general block diagram of a transmission system, accordingto certain aspects of the embodiments of this disclosure;

FIG. 1B is a block diagram of additional detail related to photonicintegrated circuits of the transmission system of FIG. 1A;

FIG. 2A is a diagrammatic side view of a photonic integrated circuitoutput carrier frequency shift due to a change in temperature;

FIG. 2B is a representative example of the carrier frequency shiftassociated with the photonic integrated circuit of FIG. 2A.

FIG. 2C is a depiction of the carrier frequencies of outputs signalsfrom four photonic integrated circuits prior to the optical outputsignals being combined;

FIG. 2D is a representative example of the combined optical outputsignals of FIG. 2C.

FIG. 2E is another representative example of another combined opticaloutput, according to certain aspects of the embodiments of thisdisclosure;

FIG. 2F is yet another representative example of another combinedoptical output, according to certain aspects of the embodiments of thisdisclosure;

FIG. 3A is a block diagram of an exemplary carrier frequency controllerprocessing circuit, according to certain aspects of the embodiments ofthis disclosure;

FIG. 3B is a block diagram of additional detail related to theprocessing circuit of FIG. 3A;

FIG. 3C is an exemplary representation of a signal output at a firstpoint within the carrier frequency controller processing circuit of FIG.3A;

FIG. 3D is a signal output at a second point within the carrierfrequency controller processing circuit of FIG. 3A;

FIG. 4A is a depiction of exemplary frequency grids which may beprocessed by the exemplary carrier frequency controller processingcircuit of FIG. 3A;

FIG. 4B is an exemplary representation of the frequency grids of FIG. 4Aprocessed by the carrier frequency controller processing circuit of FIG.3A; and

FIG. 5 is a method of processing carrier frequency grids to maintainproper channel spacing between the various carrier frequencies,according to certain aspects of the embodiments of this disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure provides a system, apparatus and method toprovide for monitoring of characteristics of optical signals, as part ofwavelength division multiplexed signals for example, transmitted over anetwork infrastructure. The characteristics of the optical signals maybe monitored and maintained at desired values in order to optimizesystem performance. A system including a coherent detector, as part of acoherent receiver for example, may be employed to associate eachtransmitted optical signal with a modulated source. Control signalsgenerated by the system can then be provided to elements of themodulated source to control characteristics, such as optical power,optical frequency, and optical phase, for example, of the transmittedoptical signal. Such elements may include, but are not limited to, alaser, a modulator, an amplitude varying element, a thermal deviceconfigured to control a temperature of the laser. Characteristics of thetransmitted optical signal may be maintained to overcome impairmentsassociated with the network infrastructure over which the optical signalis transmitted.

The following description is set forth for purpose of explanation inorder to provide an understanding of the various embodiments of thedisclosure. However, it is apparent that one skilled in the art willrecognize that these embodiments, some of which are described below, maybe incorporated into a number of different systems and devices.Additionally, the embodiments of the present disclosure may includecertain aspects each of which may be present in hardware, software, orfirmware. Structures and devices shown in block diagram in the figuresare illustrative of exemplary embodiments and are meant to avoidobscuring certain aspects of the embodiments of the disclosure.Furthermore, connections between components within the figures are notintended to be limited to direct connections. Rather, data between thesecomponents may be modified, re-formatted or otherwise changed byintermediary components.

FIG. 1A illustrates an exemplary transmitter or transmission system 100including a number, N, of photonic integrated circuits (PICs) 110-1through 110-N, collectively referred to as photonic integrated circuits110 or PICs 110. As discussed in greater detail with respect to FIG. 1Bbelow, each of the PICs 110 provides a corresponding optical outputsignal, identified as optical output signals 130-1 through 130-N,collectively referred to as optical output signals 130. Each of theoptical output signals 130 may include a number of optical signals, eachhaving a different carrier frequency, as discussed in greater detailbelow with respect to FIG. 1B. The carrier frequencies, as part of eachof the output signals 130, define a frequency grid, identified as FG-1through FG-N, collectively referred to as frequency grids FG. Theoptical output signals 130 may then be provided to an optical combiner132 which combines the signals 130 into a combined output signal 133,such as a WDM signal, suitable for transmission. As part of the combinedoutput signal 133, each of the channels of each of the optical outputsignals 130 is spectrally spaced from adjacent channels, referred to aschannel spacing. The optical combiner 132 may then combine the receivedoptical outputs 130 into a combined output signal 133. Optical combiner132 may be any suitable optical combiner, such as a power combiner or aflexible wavelength muxing combiner, where the combiner can be tuned tomatch the incoming carrier frequencies of the optical signals 130.

The combined output signal 133 may then be provided to an optical tap134 which may direct a majority of the combined output signal 133 topass as output signal 136, for transmission over an optical network 137for example, while diverting a signal 138, which is a portion of thecombined output signal 133, to a carrier frequency controller circuit140. The carrier frequency controller 140, as discussed in more detailwith respect to FIG. 3A, may provide a plurality of control signals 142to the PICs 110, e.g. control signals 142-1 through 142-N. In this way,carrier frequency controller 140 may detect the carrier frequencies ofthe signals 130, as part of combined signal 133, and provide controlsignals 142 to maintain proper channel spacing, as is discussed ingreater detail with respect to FIG. 1B.

While FIG. 1A illustrates the origin of the optical signal 138 as beingpart of the transmission system 100, the origin of the optical signal138 may be elsewhere. For example, a remote tap 134R may divert aportion of the transmitted signal 136, and supply the portion to thecarrier frequency controller 140 processing circuitry as signal 138R1.In that case, the signal 138R1 may be provided to the carrier frequencycontroller 140 via transmission link 137, or through another opticalline or path 137R1, as depicted. Alternatively, the wavelengthcontroller 140 may be remotely located with respect to the transmissionsystem 100, as represented by optional carrier frequency controller 140R(in dashed line). In this alternative approach, a portion of thetransmitted optical signal 136 may be provided to the optional carrierfrequency controller 140R as a signal 138R2. carrier frequencycontroller 140R may then provide control signals 142R back to thetransmission system 100, over path 137R2 for example.

The control signals 142R may be directed to the PICs 110 as representedby control signals 142 or, alternatively, to an optional signalconditioner 144 which may include circuitry to process, or providesignal conditioning to, the control signals 142R. For example, thecontrol signals 142R may be optical or electrical digital signals whichrepresent desired changes with respect to the output carrierfrequencies, and ultimately the corresponding channel spacing,associated with optical signals 130. Thus, the control signals 142R maybe processed by optional signal conditioner 144 to determine whichcarrier frequencies are to be modified. Alternatively, control signals142R may be analog signals which represent desired changes with respectto the output carrier frequencies, as part of optical signals 130. Theoptional signal conditioner 144 may include circuitry which conditionsthe analog signals by filtering undesired signals from the controlsignals 142R or providing a desired gain to the control signals 142R.

While FIG. 1A depicts a tap 134 (or tap 134R) to direct a portion 138 ofthe output signal 133 to the carrier frequency controller 140, anoptical switch (not shown) may be used in place of tap 134. The opticalswitch may be used to switch the output signal 133 and direct the signal133 to the carrier frequency controller 140, as signal 138 for example,at various time periods to ensure the carrier frequencies of the outputsignals 130 are maintained at desired channel spacings. Such timeperiods may include, for example, initial system power-up andinitialization, a specific time intervals during operation, or when anoperational aspect of the transmission system 100 changes. Suchoperational aspects may include, but are not limited to, modulationformat changes of one or more PIC 110 outputs 130, link 137 changeswhich result in a corresponding change in a transmission distance, or achange in a capacity requirement associated with link 137. Also,additional optical elements may be included throughout the transmissionsystem 100, if desired, to provide additional amplification orattenuation of one or more of the signals 130, 133, 136, 138, 142, forexample.

Turning to FIG. 1B, each PIC 110 will be discussed in greater detail.FIG. 1B depicts a block diagram of a first PIC 110-1 of the transmissionsystem 100, each remaining PIC 110-2 through 110-N may be similar to PIC110-1. PIC 110-1 may include a number of signal channels, signalchannels 112-1 through 112-M for example, collectively referred to assignal channels 112. Each of the signal channels 112 may include a lightsource 114, an optional amplitude varying element (AVE) 116 (in dashedline) and a modulator 118. The light source 114 may be any suitablelight source, such as a distributed feedback (DFB) laser or adistributed Bragg reflector (DBR) laser for example. The light source114 provides light 115 at a desired wavelength, within the C-Band rangeof 1525 nm to 1565 nm for example. The light 115 is provided to themodulator 118, optionally amplified or attenuated by the AVE 116, ifpresent. For example, the light may be amplified by the AVE 116 suchthat the light 115 each of the signal channels 112 have similar opticalpowers, or otherwise have a desired power profile across the signalchannels 112 where each optical power associated with each of the signalchannels 112 may be similar or different from optical powers of theremaining signal channels 112.

The modulator 118 modulates the light 115 in accordance with a datastream to provide a modulated output 120 representative of the datastream once received and demodulated in a remote receiver, as is knownin the art. The modulator 118 may be any suitable modulator, such as aMach-Zehnder modulator or an electro-absorption modulator, whichmodulates the corresponding light 115, preferably in a phase-shiftkeying modulation format, such as binary phase-shift keying (BPSK)format or quadrature phase-shift keying (QPSK) format, for example. If apolarization multiplexed modulation format is preferred, modulator 118may be a first of two modulators 118A, 118B (not shown) per channel 112.The first modulator 118A may supply a first modulated signal 120A whichwill be associated with a first polarization state or mode, such as aTransverse Electric (TE) or a Transverse Magnetic (TM) mode. A secondmodulator 118B may be provided to supply a second modulated signal 120which will be associated with a second polarization state or mode. Moreinformation regarding polarization multiplexed transmission systems maybe found in copending application Ser. No. 12/646,952 entitled“Transmitter Photonic Integrated Circuit” and Ser. No. 12/572,179entitled “Coherent Optical Receiver”, both of which are incorporatedherein by reference in their entirety.

Each of the signal channels 112 may then provide an associated modulatedoutput 120, e.g. 120-1 through 120-M, to an optical combiner 126 whichcombines the modulated outputs 120 into the output signal 130-1, andprovided as an output of the PIC 110-1. The optical combiner 126 may beany suitable combiner, such as a power combiner which will combine theoptical signals without reference or consideration of the wavelengths ofeach light 115 associated with the modulated outputs 120 of each signalchannel 112. Alternatively, optical combiner 126 may be a wavelengthselective combiner, such as an arrayed waveguide grating (AWG). An AWGmay provide a lower loss multiplexing structure with respect to a powercombiner, while providing a narrow passband for the modulated outputs120 which may aide in filtering undesirable optical noise. If an AWG isused as the optical combiner 126, a thermal device 126-T, such as athermal electric cooler or a heater, may be used to shift the passbandof the AWG 126 in order to ensure that the bandwidth of the modulatedoutputs 120 is positioned within the passband of the AWG 126. If aheater is employed as thermal device 126-T, such heater 126-T may be,for example, a resistive thin film heater, such as a platinum heater.The thermal device 126-T may be controlled by frequency controller 140,or other device as part of PIC 110-1, or external to PIC 110-1.

As shown, each signal channel 112 may also include a thermal device114-T, such as a thermal electronic cooler or a heater, positionedwithin thermal contact of the light source 114. A heater 114-T may be,for example, a resistive thin film heater, such as a platinum heater.Thermal energy provided by the thermal device 114-T may provideadjustment of the wavelength or carrier frequency of the light 115exiting the light source 114. Thus, the carrier frequency of each light115 provided by each light source 114 of each signal channel 112 may beindependently controlled to achieve a desired PIC signal channel spacingwhich, in turn, defines an associated frequency grid. Such frequencygrid signal channel spacing may be, for example, 100 GHz or 200 GHz.Control of the thermal device 114-T may be provided by the carrierfrequency controller 140 circuit, as discussed below in greater detailwith respect to FIG. 3A. The individual carrier frequencies associatedwith each light source 114 may also be controlled through a bias signalprovided to the light source 114, as is known in the art. The biassignal may be a bias voltage or current, and may be provided, as leastin part, by the carrier frequency controller 140. Alternatively, thermalenergy from thermal device 114-T and the bias signal may be provided toeach laser source 114, in combination, to control the carrier frequencyof the source 114. Such control signals may be provided, as least inpart, by the carrier frequency controller 140 in accordance with thepresent disclosure.

The PIC 110-1 may also include thermal device 122-T which is positionedto be in thermal contact with each of the signal channels 112,collectively. The thermal device 122-T may then be used to apply thermalenergy to the light sources 114 to adjust the carrier frequencies ofeach of the light sources 114, collectively. Such adjustment allows forthe shifting of the associated frequency grid, with respect to astandardized grid for example. The thermal device 122-T may be a heateror a thermal electronic cooler. A heater 122-T may be, for example, aresistive thin film heater, such as a platinum heater. Preferably, eachof the thermal devices 114-T are heaters and the thermal device 122-T isa thermal electric cooler. Thus, while each of the thermal devices 114-Tprovide individual carrier frequency adjustments to each correspondinglaser source 114, the thermal device 122-T may provide overalltemperature changes collectively to all the light sources 114 to shiftthe output carrier frequency grid FG-1 of the output optical signal130-1. In this way, each of the frequency grids FG of each of the outputsignals 130, provided by a corresponding one of the PICs 110, may bespectrally shifted to provide a desired channel spacing. Such carrierfrequency grid shifting may be provided dynamically to continuouslyobtain a desired channel spacing to achieve higher capacity or longerreach, for example. Additional information regarding the fabrication ofPICs 110 may be found in U.S. Pat. No. 7,283,694 entitled “TransmitterPhotonic Integrated Circuits (TxPIC) and Optical Transport EmployingTxPICs,” which is incorporated herein by reference in its entirety.

With reference to FIG. 2A-2D, the dynamic shifting of the carrierfrequency grids FG of each of the PICs 110 is described in greaterdetail. FIG. 2A depicts a PIC 110-1 which provides an output signal130-1 including four modulated optical signals 120-1 through 120-4, eachat a different carrier frequency f_(1A)-f_(4A), respectively. The outputsignal 130-1 is described as having four modulated signals 120 forillustration purposes only. The output signal 130-1 may include more orless modulated signals 120, each at a different carrier frequency. ThePIC 110-1 includes thermal devices 114-T (as shown in FIG. 1B) whicheach provide thermal energy to respective light sources 114 to tune theassociated individual carrier frequencies f_(1A)-f_(4A) of the outputsignal 130-1, to obtain a desired channel spacing for example. The PIC110-1 also includes a thermal energy device 122-T positioned to supplythe PIC 110-1 with thermal energy such that the PIC 110-1 is at a firsttemperature T1. Thus, at the first temperature T1, the PIC 110-1provides the output signal 130-1 including four modulated signals 120 ofcarrier frequencies f_(1A)-f_(4A), respectively, the carrier frequencyf_(1A)-f_(4A) defining a frequency grid FG-1A. Frequency grid FG-1A mayalso be referred to as an optical channel group (OCG), e.g. OCG-1. Thethermal device 122-T may be controlled, through a signal 142-1T from thecarrier frequency controller 140 as shown in FIG. 1B for example, toprovide a different thermal energy to the signal channels 112 of PIC110-1. For example, the carrier frequency controller 140 may provide avoltage or current to the thermal device 122-T, the thermal energy fromthe thermal device 122-T provided in response to the voltage or current.The different thermal energy results in a temperature change ΔT of PIC110-1 from the first temperature T1 to a second temperature T2. Inresponse to the temperature change ΔT, the frequency grid FG-1A of theoutput signal 130-1, spectrally shifts such that the four modulatedsignals 120 have carrier frequencies f_(5A)-f_(8A), respectively. FIG.2B illustrates the frequency grid FG-1A including carrier frequenciesf_(1A)-f_(4A) of output signal 130-1 corresponding to the PIC 110-1 (PIC1) at the first temperature T1. Each of the carrier frequenciesf_(1A)-f_(4A) are depicted along axis f. The frequency grid FG-1A mayinclude a frequency grid channel spacing f_(G) between each of thecarrier frequencies f_(1A)-f_(4A). FIG. 2B also depicts the outputsignal 130-1 of PIC 110-1 at the second temperature T2. In response tothe temperature change from T1 to T2 the frequency grid FG-1A hasspectrally shifted an amount f_(T), the frequencies f_(5A)-f_(8A) (indashed line) representing the carrier frequencies associated with outputsignal 130-1 of PIC 110-1 at the second temperature T2. The amount f_(T)the carrier frequency grid FG-1A has spectrally shifted is forillustration purposes only. For example, the amount f_(T) may be lessthan the channel spacing f_(C).

FIG. 2C is an exemplary representation of four PIC devices, PIC 1-4being similar to PIC 110-1 for example, each having frequency grids FGsincluding four of the carrier frequencies f_(1B) through f_(16B). Thus,PIC 1 provides an output signal 130-1 having a frequency grid FG-1Bincluding carrier frequencies f_(1B) through f_(4B) defining OCG-1, PIC2 provides an output signal 130-2 (shown in dashed line) having afrequency grid FG-2B including carrier frequencies f_(5B) through f_(8B)defining OCG-2, PIC 3 provides an output signal 130-3 having a frequencygrid FG-3B including carrier frequencies f_(9B) through f_(12B) definingOCG-3, and PIC 4 provides an output signal 130-4 (shown in center line)having a frequency grid FG-4B including carrier frequencies f_(13B)through f_(16B) defining OCG-4. As discussed above, each of theexemplary PIC devices, PIC 1-4, may include thermal devices 114-T fortuning individual channel spacings, as well as the thermal device 122-Tto shift the particular frequency grid, e.g. FG-1B through FG-4B, todefine the outputs as depicted in FIG. 2C. For illustration purposesonly, each of the frequency grids FG-1B to FG-4B is shown as having afrequency grid channel spacing of 200 GHz, however other suitablefrequency grid channel spacings are contemplated herein, such as a 100GHz channel spacing. The four frequency grids FG-1 to FG-4B may or maynot conform to an associated standardized wavelength grid, such as anITU standardized grid. The four PIC 110 output signals 130-1 through130-4 may then be combined, with optical combiner 132 of FIG. 1A forexample, to supply the output signal 133A as illustrated in FIG. 2D.

As shown in FIG. 2D, the combined output signal 133A includes thefrequency grids FG-1B through FG-4B of each of the PICs 110-1 through110-4, respectively. The frequency grids FG-1B through FG-4B overlapsuch that a portion of each frequency grid overlaps a portion of theremaining frequency grids. Each of the PICs 110-1 through 110-4 aretuned, through operation of thermal devices 114-T, 122-T for example, toprovide their corresponding signal channels, e.g. f₁ through f₄associated with FG-1B, having individual channel spacing within eachfrequency grid FG-1B through FG-4B of 200 GHz, as shown in FIG. 2C.Turning back to FIG. 2D, each of the signal channels associated withfrequency grids FG-1 through FG-4, however, are offset 50 GHz withrespect to adjacent signal channels, e.g. have channel spacings of 50GHz.

While only the outputs of four PICs 110 are illustrated in the exemplaryrepresentation of FIGS. 2C and 2D, the corresponding transmission system100 may include 10 PICs 110 (e.g. N=10). In this case, only four of theten PICs 110 are deployed, the remaining six PICs 110 not being deployedin this scenario. FIGS. 2E and 2F provide exemplary representations ofcombined signals 133 where increasing capacity is desired. The capacityof the transmission system 100 can be increased by utilizing eight PICs110 instead of the four PICs 110 contemplated in the example of FIGS. 2Cand 2D, each of the eight PICs providing an output 130 including tensignal channel outputs 120. In response to control signals 142, forexample, eight PICs 110, each providing ten signal channel outputs 120,may be deployed in the system 100, and the channel spacing may changeaccordingly. As illustrated in FIG. 2E, the combined output signal 133Bfrom the eight PICs 110 of the transmission system 100 would includeeight outputs 120, each associated with ten carrier frequencies. Theinterleaved output signal 133B would include frequency grids FG-1Cthrough FG-8C. The three overlapping frequency grids FG-1C, FG-2C, andFG-8C are partially illustrated. For example, frequency grid FG-1C mayinclude frequencies f_(1C) through f_(10c), frequency grid FG-2C mayinclude frequencies f_(11C) through f_(20C), and frequency grid FG-8Cmay include frequencies f_(71C) through f_(80C). The optical signals 120associated with one of the frequency grids FG-1C through FG-8C may bemodulated in accordance with a first modulation format, or to carry dataat a first data rate, while the optical signals 120 associated with oneor more of the remaining of the frequency grids FG-1C through FG-8C maybe modulated in accordance with a second modulation format, or to carrydata at a second data rate. The optical signals 120 associated with oneof the frequency grids FG-1C through FG-8C may have a first channelbandwidth while optical signals 120 of one or more of the remainingfrequency grids FG-1C through FG-8C may have a second channel bandwidth.In response to first control signals 142, a first signal channel 11C offrequency grid FG-2C having a frequency of f_(11C) may be spectrallyspaced 25 GHz from a first signal channel 1C of frequency grid FG-1Chaving a frequency of f_(1C), and 175 GHz from a second signal channel2C of frequency grid FG-1C having a frequency of f_(2C).

The individual single channel spacing, e.g. spectral spacing between thefrequency f_(1C) and f_(11C), as well as frequency grid spacing, e.g.the spectral offset between frequency grid FG-1C and frequency gridFG-2C, may be monitored and maintained through signal processing of theoutput signal 133B by the carrier frequency controller 140 circuitry, asdescribed in greater detail with respect to FIG. 3 below. The channelspacing between adjacent channels of interleaved output 133B may becalculated by dividing the frequency grid channel spacing, e.g. 200 GHz,by the number of PICs 110 utilized, in this case eight PICs 110. Whileproviding higher channel 112 densities, the output signal 133B may bemore susceptible to certain transmission impairments, such as non-linearimpairments for example, which may limit the reach of the transmission.

Higher capacity can be achieved by utilizing all ten PICs of thetransmission system 100, an interleaved output signal 133C shown in FIG.2F. The combined output signal 133C would include 100 carrierfrequencies. As depicted, the output signal 133C may include the tenfrequency grids FG-1D through FG-10D, each including ten signal channels120. The three overlapping frequency grids FG-1D, FG-2D, and FG-10D arepartially illustrated. For example, frequency grid FG-1D may includefrequencies f_(1D) through f_(10D), frequency grid FG-2D may includefrequencies f_(11D) through f_(20D), and frequency grid FG-10D mayinclude frequencies f_(91D) through f_(100D). The optical signals 120associated with one of the frequency grids FG-1D through FG-10D may bemodulated in accordance with a first modulation format, or to carry dataat a first data rate, while the optical signals 120 associated with oneor more of the remaining of the frequency grids FG-1D through FG-10D maybe modulated in accordance with a second modulation format, or to carrydata at a second data rate. Also, the optical signals 120 associatedwith one of the frequency grids FG-1D through FG-10D may have a firstchannel bandwidth while optical signals 120 of one or more of theremaining frequency grids FG-1D through FG-10D may have a second channelbandwidth. As compared with signal 133B of FIG. 2E, in response tosecond control signals 142 ten PICs 110 may be deployed, each providingan output 130 including ten signal channel outputs 120. With thefrequency grid FG channel spacing maintained at 200 GHz, the increase inoverall signal channels 120 from 80 signal channels in output signal133B to 100 signal channels in output signal 133C, will result in achange in the channel spacing. For example, a first signal channel 11Dof frequency grid FG-2D having a frequency of f_(11D) may be spectrallyspaced 20 GHz from a first signal channel 1D of frequency grid FG-1Dhaving a frequency of f_(1D), and 175 GHz from a second signal channel2D of frequency grid FG-1D having a frequency of f_(2D). Thus, thechannel spacing between individual channels as part of the output signal133 would change from 25 GHz, as shown in FIG. 2E, to 20 GHz, as shownin FIG. 2F.

The frequency grid, such as the frequency grid of carrier frequenciesf_(1C) through f_(80C) of output signal 133B as depicted in FIG. 2E orcarrier frequencies f_(1D) through f_(100D) of output signal 133C asdepicted in FIG. 2F, may be generalized as follows:

${f_{Grid} = \left. {f_{Initial} + {m*\frac{\Delta \; F}{N}} + {M*\Delta \; F}} \middle| {M \in \left\{ {0,1,{2\mspace{14mu} \ldots \mspace{14mu} 9}} \right\}} \right.},{m \in \left\{ {0,1,{{2\mspace{14mu} \ldots \mspace{14mu} N} - 1}} \right\}}$

where:

-   -   f_(Initial)=initial frequency of the frequency grid;    -   ΔF=frequency grid channel spacing;    -   N=number of PICs employed; and    -   M=number of channels per PIC.        Thus, the frequency grid of FIG. 2E may be defined as having an        initial frequency, f_(Initial) of 193.950 THz, a frequency grid        channel spacing, ΔF, of 200 GHz, a number of PICs (OCGs)        deployed equaling 8, and a number, n, of channels per PIC        being 10. The resulting channel spacing would be

$\frac{\Delta \; F}{N},$

or 200 GHz/8, or 25 GHz, as depicted in FIG. 2E. Similarly, thefrequency grid of FIG. 2F may be defined as having an initial frequency,f_(Initial), of 193.950 THz, a frequency grid channel spacing, ΔF, of200 GHz, a number of PICs (OCGs) deployed equaling 10, and a number, n,of channels per PIC being 10. The resulting channel spacing would be

$\frac{\Delta \; F}{N},$

or 200 GHz/10, or 20 GHz, as depicted in FIG. 2F.

Turning to FIG. 3A, an exemplary carrier frequency controller 140 isdepicted. The carrier frequency controller 140 of FIG. 3A is configuredto monitor the individual signal channel 112 carrier frequencies, aspart of frequency grids FGs associated with PIC 110 optical outputsignals 130, and adjust the signal channel 112 carrier frequencies asnecessary to achieve desired channel spacing in the combined outputsignal 133. As discussed above with reference to FIG. 1A, the carrierfrequency controller 140 may receive a portion of the combined outputsignal 133 as the signal 138. The signal 138 may include the frequencygrids FG which, in turn, include individual signal channel 112 carrierfrequencies f₁-f_(M) for each of the number, N, of PICs 110. Thus, thesignal 138 may include carrier frequencies 138 _(f), represented by f₁₋₁through f_(N-M), where N is the number of PICs 110 and M is the numberof signal channels 112 per PIC 110. The carrier frequency controller 140may include an optical processing circuit 350, a carrier frequencyprocessing circuit 364 and a local oscillator circuit 365. The localoscillator circuit 365, for example, may include a local oscillator 366,an etalon 370 and a photodetector 372, such as a photodiode.

The optical processing circuit 350 may include a coherent detector orreceiver circuit, as is known in the art. Turning to FIG. 3B, theoptical processing circuit 350 may include a 90-Degree optical hybrid352 circuit configured to receive two input signals 351 and supply fouroutput signals 353 in response to the received signals 351. For example,a first signal 351A including a carrier frequency f_(1E) and a secondsignal 351B including a frequency f_(2E), typically provided by anoptical oscillator local to the optical hybrid 352, are received by theoptical hybrid 352. As is known in the art, the optical hybrid 352 mixesthe incoming signals 351 to provide the four output signals 353. A firstpair 353-1A, 353-1B of outputs 353 may be provided to a first balancedpair of detectors 364A which converts the first pair 353-1 of outputs353 into a corresponding electrical signal 355A. Similarly, a secondpair 353-2A, 353-2B of outputs 353 may be provided to a second balancedpair of detectors 364B which converts the second pair 353-2 of outputs353 into a corresponding electrical signal 355B. The signal 355A mayrepresent the in-phase component of the signal 355, while the signal355B may represent the quadrature component of the signal 355. Each ofthe signals 355A, 355B are provided to corresponding transimpedanceamplifiers/automatic gain controllers 356A, 356B and analog to digitalconverters 358A, 358B, respectively, prior to being provided to adigital signal processor (DSP) 360 as signals 359. As is known in theart, the DSP 360 may be configured to apply signal process algorithms tothe signals 359 to provide an output 361 from which the amplitude andphase of the input signal 351A may be determined, and ultimately thedata encoded within input signal 351A, if desired.

When the frequency f_(2E) of the input signal 351B is close to thecarrier frequency f_(1E) of the input signal 351A, output 361 isgenerated by optical processing circuit 350. That is, when thefrequencies f_(1E) and f_(2E) are close to the same value, opticalmixing in the optical hybrid 352 results in an output 353. Outputs 353are further processed by the optical processing circuit 350 to generateoutput 361. The DSP 360 may be configured to provide an output signal362 indicative of the frequency f_(2E) of signal 351B being close to thecarrier frequency f_(1E) of signal 351A. Thus, when the frequenciesf_(1E), f_(2E) are close, the output signal 362 may be provided toinclude an amplitude, A, at a carrier frequency corresponding to thecarrier frequency f_(1E), and when the frequencies f_(1E), f_(2E) arenot close, the output signal 362 may have an amplitude less than A. Ifthe frequency f_(2E) of the input signal 351B is configured to scan orsweep through an associated frequency range f_(Range) of, e.g. fromf_(S-Min) through f_(S-Max), the output signal 362 will indicate thecarrier frequencies 138 _(f) present in the signal 351A which are withinthe associated frequency range f_(Range). The input signal 351B may beconfigured to sweep through the range of frequencies f_(Range) whenverification of the carrier frequencies 138 _(f) is desired, formaintaining proper channel spacing for example. Alternatively, the inputsignal 351B may be further configured to continuously sweep through therange f_(Range) to continuously monitor and maintain the channelspacing, or other characteristics, associated with the signals 120. Suchan output signal 362 is depicted in FIG. 3C, where the carrierfrequencies present in the signal 138 are identified as carrierfrequencies 138 _(λ). The carrier frequencies 138 _(f) are within theassociated range of frequencies f_(Range), e.g. from f_(S-Min) tof_(S-Max). Turning back to FIG. 3A, the output signal 362 from theoptical processing circuit 350 may be provided to the carrier frequencyprocessing circuit 364, and processed as discussed in greater detailbelow.

The output signal 362 may provide an indication of the carrierfrequencies 138 _(f) present in the input signal 138. Carrierfrequencies 138 _(f) are preferably associated with the carrierfrequencies of the individual signal channels 112 of the transmissionsystem 100. The oscillator circuit 365 provides a reference from whichthe carrier frequencies 138 _(f) can be compared to associate each oneof the carrier frequencies with a corresponding PIC 110. Once thecarrier frequencies 138 _(f) are associated with their PIC 110, thechannel spacing of the output signals 130 may be maintained at desiredlevels, as described in greater detail above.

The oscillator circuit 365 includes the local oscillator 366 whichsupplies first light 367 at a frequency f_(Scan) to the opticalprocessing circuit 350, as input 351B for example. The local oscillator366 may be a laser source, for example a DFB laser or a DBR laser. Thelocal oscillator 366 is configured to provide the first light 367 at afrequency f_(Scan) which scans or sweeps through an associated range offrequencies f_(S-Range), f_(S-Min) through f_(S-Max) for example, at aknown sweep rate f_(S-Rate). The frequency f_(Scan), also referredherein as scanning frequency f_(Scan), may sweep in a single directionfrom f_(S-Min) to f_(S-Max) or from f_(S-Max) to f_(S-Min).Alternatively, the frequency f_(Scan) may repeatedly sweep in the singledirection, or back and forth from f_(S-Min) to f_(S-Max) to f_(S-Min),for example. The range of frequencies f_(S-Range) is selected such thatthe carrier frequencies 138 _(f) present in the signal 138 areassociated with a bandwidth which is within the range of frequenciesf_(Scan). Thus, while providing the signal 138 as the first input 351Asignal and the first light 367 at the scanning frequency f_(Scan) as thesecond input 351B signal, the optical processing circuit 350 wouldprovide an output 362 which would identify the carrier frequencies 138_(f) present in the signal 138.

Local oscillator 366 may provide a second light 368A at the frequencyf_(Scan), e.g. the scanning frequency f_(Scan), to the etalon 370.Alternatively, the oscillator circuit 365 may include optional tap 369(in dashed line) which may be configured to receive the first light 367from the oscillator 366 and divert a portion of the light 367 as light368B (in dashed line) to the etalon 370, the remainder of the firstlight provided to the optical processing circuit 350 for example. If thelight 368B is used, the light 368A would not be necessary. The etalon370 provides an output 371 to photodetector 372 which converts thereceived optical signal 371 into a corresponding electrical signal 374,which is then provided to carrier frequency processing circuit 364 forfurther processing. The etalon 370, as known in the art, may include aresonant structure configured to provide an output signal at a knownamplitude if the wavelength of the associated frequency f_(Scan) thelight 368A resonates with respect to the resonant structure of theetalon 370. FIG. 3D depicts an exemplary output 374, as provided by theetalon 370 and photodiode 372 for example, over the second light 368Afrequency f_(Scan) associated with the frequency range of f_(S-Min) tof_(S-Max). As the frequency of second light 368A scans through thefrequency range, when the associated frequency f_(Scan) is close to aresonant frequency f_(E) of the etalon, an output at the correspondingfrequency f_(E) is provided by etalon 370. Thus, the output 374, asdepicted, represents the output of etalon 370 over the entire range offrequencies f_(S-Range). Since the output 374 has a specific amplitudewhich differs with respect to the range of frequencies f_(E-Range), fromf_(E-Min) to f_(E-Max) as shown, the frequency associated with thefrequency f_(Scan), when close to one of the frequencies f_(E), isknown. The etalon 370 has a frequency range of f_(E-Range), for exampleextending from f_(E-Min) to f_(E-Max), the range f_(E-Range) beingselected such that the frequencies associated with carrier frequencies138 _(f) present in signal 138 are within the range f_(E-Range), asdepicted. Since the etalon 370 provides outputs at specific frequenciesover the range of frequencies f_(Scan) present in the light 368A, etalon370 may be considered an optical filter which passes only selected knownfrequencies f_(E).

The carrier frequency processing circuit 364 may superimpose the signal362 from optical processing circuit 350 over the known frequencies f_(E)generated by the output 374 from the etalon 370. Since the frequencyassociated with frequencies f_(Scan) of second light 368A sweeps at aknow rate f_(S-Rate), and the etalon 370 provides an output at knownfrequencies f_(E), as provided by signal 374, carrier frequencies 138_(f) can be associated with the individual PICs 110 to define associatedfrequency grids FG within the frequencies 138 _(f). Once the frequencygrids FG have been associated with their originating PICs 110, theindividual channel 112 spacing of each PIC 110, can be controlled or setto a desired amount by the carrier frequency processing circuit 364through the output signals 142, e.g. the control signals 142, of thecarrier frequency controller 140, as discussed above with respect toFIG. 1A. For example, the carrier frequency processing circuit 364 candetermine a time period from an intensity peak of the etalon 370 outputf_(E) and the detection of a signal 362, the signal 362 indicative of arespective one of a plurality of carrier frequencies 138 _(f). The timeperiod can then be multiplied by the sweep rate f_(S-Rate) to provide afrequency offset, which can then be added to the intensity peakfrequency to provide the frequency of interest, e.g. the frequencyassociated with the detected signal 362.

Since the carrier frequency controller 140 can associate individualsignal channel frequencies with signal channels 112 of the PICs 110, inaddition to controlling the channel spacing, the carrier frequencycontroller 140 may be employed in controlling other characteristics ofthe transmission system 100, or the components which make up thetransmission system as described and contemplated herein. Thecharacteristics may be monitored and maintained at desired levels,through application of the control signals 142 for example, to enhanceor compensate the performance of the transmission system, in light ofany impairments which may be present. Such characteristics may berelated to the optical signals 120 provided by the signal channels 112,such as a laser 114 bias signal, a modulator 118 bias signal, or a gainprovided by the AVE 116, to control a power of the output signal 120 forexample.

Turning to FIGS. 4A and 4B, an example of dynamically controlling thesignal channel 112 spacing of a combined signal such as signal 138including a plurality of carrier frequencies, utilizing the carrierfrequency controller 140 of FIG. 3 will be discussed in greater detail.For purposes of simplicity, only selected carrier frequencies of theoutput of only two PICs 110 will be described. FIG. 4A depicts aninterleaved output from two PICs 110, PIC 1 and PIC 2 (in dashed line).In accordance with the present disclosure, PIC 1 supplies an output130-1 which includes a number of signal channel outputs, each at arespective one of eight carrier frequencies identified as carrierfrequencies f_(1F) through f_(8F). The carrier frequencies f_(1F)through f_(8F) define a frequency grid FG-1, each of the signal channeloutputs of PIC 1 being spaced by an amount f_(G), which may be 100 GHzor 200 GHz for example. Similarly, PIC 2 may supply an output 130-2 (indashed line) which includes a number of signal channel outputs, each ata respective one of eight carrier frequencies identified as carrierfrequencies f_(9F) through f_(16F). The carrier frequencies f_(9F)through f_(16F) may define a frequency grid FG-2, each of the signalchannel outputs of PIC2 being spaced by an amount f_(G), which may be100 GHz or 200 GHz for example. The individual channel 112 spacingsbetween each channel of frequency grid FG-1 and an adjacent signalchannel of frequency grid FG-2 is represented by f_(C) In accordancewith the above discussion regarding transmission system 100, the signal130-1 may be interleaved, or otherwise combined, with signal 130-2 byoptical combiner 132 to form a combined output signal 133. A portion ofthe output signal 133 is diverted to the carrier frequency controller140, as the signal 138 for example. Signal 138 thus includes individualsignals having carrier frequencies f_(1F) through f_(16F). In accordancewith the discussion above, the optical processing circuit 350 willprovide an output 362 over an associated frequency range f_(Scan) asdepicted in FIG. 3C, the output 362 indicative of the carrierfrequencies present in the signal 138, as represented by 138 _(f).Additionally, the oscillator circuit 365 of wavelength controller 140provides a signal 374 which includes defined outputs at associatedfrequencies f_(E) over a portion of the frequency range f_(S-Range),from f_(S-Min) to f_(S-Max) for example, as depicted in FIG. 3D. Thecarrier frequency processing circuit 164 then processes the signals 362and 374 to provide control signals 142 to maintain the proper channelspacing f_(C) and frequency grid channel spacing f_(G). Morespecifically, as illustrated in FIG. 4B, the carrier frequencyprocessing circuit 164 superimposes the signal 362, represented bysignals 1A-1C, and 2A-2C, over the signal 374. For simplicity, theetalon 370 provides an output 374 having six intensity peaks E₁ throughE₆, collectively referred to as output E, at corresponding frequenciesf_(E1) through f_(E6), respectively. As discussed above with respect toFIG. 3D, each of the peaks E₁ through E₆ of output E has an associatedamplitude, respectively. Portions of frequency grids FG-1, FG-2 aredepicted for simplicity, the bandwidth of each of the frequency gridsFG-1, FG-2 being within the bandwidth of the etalon 370 output E, e.g.within the range of frequencies f_(E1) through f_(E6). Additionally, asshown, the bandwidth of the etalon 370 output E are within the frequencyrange f_(S-Range) associated with the frequency f_(Scan).

In operation, the frequency f_(Scan) sweeps through associatedfrequencies from f_(S-Min) to f_(S-Max) in the direction of arrowf_(Scan) at a known rate f_(S-Rate). When the frequency f_(Scan)associated with the frequency f_(S) is equal to a frequency f_(E)corresponding to a peak of the etalon 370 output E, an output signal 374of a specific amplitude is generated. For example, as the frequencyf_(Scan) sweeps toward f_(S-Max), at some point f_(Scan) equals f_(E1)resulting in the output peak E1 from etalon 370. As the frequencyf_(Scan) continues to scan toward f_(S-Max), frequency f_(1A) associatedwith a carrier frequency of a signal 1A of grid FG-1 is reached. Sincethe frequency f_(Scan) sweep rate f_(S-Rate) is known and the timedifference between the point where f_(Scan) equaled f_(E1) and whenf_(Scan) equaled f_(1A) can be determined, or otherwise measured, thefrequency f_(1A) associated with signal 1A can be determined. Similarly,the carrier frequencies associated with the remaining signals 1B, 1C andsignals 2A-2C can be determined.

In order to provide the proper control signals 142 to the proper PIC110-1, 110-2, the carrier frequency controller 140 needs to alsoassociate each signal 1A-1C and 2A-2C with the proper frequency gridFG-1, FG-2. The originating PIC 110 can be determined, for example,through manipulation of the frequency grids FG, e.g. spectrally shiftingone frequency grid FG at a time, or through tone assignments associatedwith the frequency grids FG. Each of the frequency grids FG-1, FG-2 maybe adjusted at known periods of time to define the frequency grids withrespect to the various signals 1A-1C and 2A-2C of FIG. 4B. For example,the thermal device 122-T of a first of the frequency grids FG-1 can beadjusted via a control signal 142-1 to PIC 110-1. In response to acorresponding temperature change, the frequency grid FG-1 willspectrally shift and, thus, the carrier frequencies f_(1A) throughf_(1C) associated with the signals 1A through 1C, respectively, willshift and their association with frequency grid FG-1 by the carrierfrequency controller 140 will be established. A similar process can beperformed regarding the frequency grid FG-2, to associate carrierfrequencies f_(2A) through f_(2C) associated with signals 2A-2C withfrequency grid FG-2. Once the individual carrier frequency associationsfor each frequency grid FG have been identified, the frequency f_(Scan)can be repeatedly scanned to monitor the individual carrier frequenciesf_(1A) through f_(1C) and f_(2A) through f_(2C), the carrier frequencycontroller 140 providing control signals 142 to the PICs 110 tocontinuously maintain proper channel spacing f_(C).

Alternatively, a different tone may be applied to each frequency grid FGof each PIC 110 output 130, as is known in the art. For example, thebias applied to the light sources 114 of a PIC may each include anoscillating signal or tone such that the amplitude is dithered inaccordance with the tone or the output wavelength of the light outputfrom the light sources 114 may be dithered in accordance with the tone.The carrier frequency controller 140 can then identify each of thefrequency grids FG, of signal 138 for example, from the associatedtones. More information regarding the use of identifying individualwavelengths, and associated carrier frequencies, of combined signals,such as signal 138, may be found in U.S. Pat. No. 7,283,694, issued Oct.16, 2007, which is incorporated by reference herein in its entirety.

Turning to FIG. 5, an exemplary method of controlling variable channelspacing, in accordance with certain embodiments described herein where achange in capacity is obtained through a change in the modulation formatemployed, will be discussed in greater detail. First and second opticalsignals are provided in a step 582. The first optical signals may beprovided by a first PCI 110-1, and the second optical signals may beprovided by a second PIC 110-2. Each of the first and second opticalsignals include channels spectrally spaced to define corresponding firstand second frequency grids, such as FG-1 and FG-2. The first and secondfrequency grids FG-1, FG-2 overlap and are spectrally spaced to define afirst channel spacing. The first and second optical signals may bemodulated in accordance with a first modulation format and combined intoan output signal in a step 584, a WDM signal for example. Upon a desiredchange in capacity, the first and second optical signals may bemodulated in accordance with a second modulation format in a step 586. Acarrier frequency controller, such as carrier frequency controller 140,may determine a second channel spacing in response to the secondmodulation format, the carrier frequency controller supplying controlsignals in a step 588. The control signal may be provided to first andsecond optical circuits associated with supplying first and secondoptical signals, respectively. For example, the frequencies associatedwith the frequency grids FG-1, FG-2 may be shifted in response to thecontrol signal in step 590 to provide for a second channel spacing. Thesecond frequency grid spacing may be different than the first frequencygrid spacing to provide necessary signal channel spacing in accordancewith the utilization of the second modulation format.

While the various embodiments of the disclosure have been described inconjunction with several specific embodiments, it is evident to thoseskilled in the art that many further alternatives, modifications andvariations will be apparent in light of the foregoing description. Thus,the embodiments described herein are intended to embrace all suchalternatives, modifications, applications and variations as may fallwithin the spirit and scope of the appended claims.

1. An apparatus, comprising: a first circuit including an oscillator anda filter, the oscillator supplying light having a frequency which sweepsat a sweep rate from a first one of a plurality of frequencies to asecond one of the plurality of frequencies, a first portion of the lightbeing provided to the filter which filters the first portion of thelight to provide a filtered output having an intensity peak at a thirdone of the plurality of frequencies; a second circuit configured toreceive a second portion of the light from the first circuit on a firstinput and an optical signal on a second input, said optical signalhaving a fourth one of the plurality of frequencies, the second circuitconfigured to provide an output signal in response to the second portionof the light and the optical signal, the output signal being indicativeof said fourth one of the plurality of frequencies; and a third circuitconfigured to provide a control signal based on, at least, the filteredoutput of the first circuit, the output signal of the second circuit,and the sweep rate, wherein the control signal is configured to controla corresponding one of a plurality of characteristics associated withsaid optical signal.
 2. The apparatus of claim 1, wherein the thirdcircuit is configured to determine a time period by measuring time fromthe intensity peak to the detection of the output signal, an offsetfrequency determined by multiplying the sweep rate by the time period,said fourth one of the plurality of frequencies being determined byadding the offset frequency to the third one of the plurality offrequencies of the intensity peak.
 3. The apparatus of claim 1, whereinthe filter includes an etalon configured to receive the first portion ofthe light and provide the filtered output as a filtered optical outputincluding the intensity peak at the third of the plurality offrequencies.
 4. The apparatus of claim 3, wherein the first circuitfurther includes a photodetector configured to receive the filteredoptical output and convert the filtered output into a filteredelectrical output, the filtered electrical output being provided by thefirst circuit as the filtered output.
 5. The apparatus of claim 4,wherein the first circuit includes an optical coupler having an inputand two outputs, the input of the coupler receiving the light, the firstportion of the light being provided at the first output of the opticalcoupler and the second portion of the light being provided at the secondoutput of the optical coupler.
 6. The apparatus of claim 4, wherein theoscillator is a laser and the light includes a first light supplied by afirst end of the laser and a second light supplied from a second end ofthe laser, the first portion of the light being the first light.
 7. Theapparatus of claim 6, wherein the oscillator is a distributed feedbacklaser.
 8. The apparatus of claim 6, wherein the oscillator is adistributed Bragg reflector laser.
 9. The apparatus of claim 1, whereinthe second circuit is a coherent detector.
 10. The apparatus of claim 9,wherein the coherent detector includes a 90-degree optical hybridconfigured to receive the optical signal and the second portion of thelight.
 11. The apparatus of claim 1, wherein the optical signal issupplied by a modulated source.
 12. The apparatus of claim 11, whereinthe modulated source includes a laser and a modulator, the opticalsignal being output from the modulator.
 13. The apparatus of claim 12,wherein the modulated source further includes an amplitude varyingelement configured to provide a gain to the optical signal.
 14. Theapparatus of claim 13, wherein the plurality of characteristics areselected from a group including an optical power of the optical signal,a phase of the optical signal, a carrier frequency of the opticalsignal, a carrier frequency of an output of the laser, a wavelength ofthe optical signal, a wavelength of the output of the laser, the gain ofthe amplitude varying element, a bias of the laser, and a bias of themodulator.
 15. The apparatus of claim 12, wherein the signal channelincludes a thermal device configured to adjust a temperature of thelaser, the temperature of the laser being set in response to the controlsignal.
 16. The apparatus of claim 15, wherein the thermal device is aheater.
 17. The apparatus of claim 15, wherein the thermal device is athermo-electric cooler.
 18. The apparatus of claim 1, wherein theintensity peak of the filtered output is a first of a plurality ofintensity peaks of the filtered output, each of the plurality ofintensity peaks being associated with a respective one of the pluralityof frequencies.
 19. An apparatus, comprising: a first circuit includingan oscillator and a filter, the oscillator supplying light having afrequency which sweeps at a sweep rate from a first of a plurality offrequencies to a second of the plurality of frequencies, a first portionof the light provided to the filter which filters the first portion ofthe light to provide a filtered output having an intensity peak at athird of the plurality of frequencies; a second circuit configured toreceive a second portion of the light from the first circuit on a firstinput and a wavelength division multiplexed optical signal on a secondinput, the wavelength division multiplexed optical signal including aplurality of optical signals, each including a corresponding one of agroup of the plurality of frequencies, the second circuit configured toprovide an output signal in response to the second portion of the lightand the wavelength division multiplexed optical signal, the outputsignal indicative of the group of the plurality of frequencies of theoptical signal; and a third circuit configured to provide a plurality ofcontrol signals in response to the filtered output of the first circuit,the output signal of the second circuit, and the sweep rate, whereineach of the plurality of control signals is configured to control acorresponding one of a plurality of characteristics associated with acorresponding one of the plurality of optical signals.
 20. The apparatusof claim 19, wherein the filter is an optical filter.
 21. The apparatusof claim 19, wherein the filter includes an etalon configured to receivethe second portion of the light and provide the filtered output as afiltered optical output including the intensity peak at the third of theplurality of frequencies.
 22. The apparatus of claim 21, wherein thefilter includes a photodetector configured to receive the filteredoptical output and convert the filtered optical output into a filteredelectrical output, the filtered electrical output being provided by thefirst circuit as the filtered output.
 23. The apparatus of claim 22,wherein the first circuit includes an optical coupler having an inputand two outputs, the input of the optical coupler receiving the light,the first portion of the light being provided at the first output andthe second portion of the light being provided at the second output. 24.The apparatus of claim 22, wherein the oscillator is a laser and thelight includes a first light supplied by a first end of the laser and asecond light supplied from a second end of the laser.
 25. The apparatusof claim 24, wherein the oscillator is a distributed feedback laser. 26.The apparatus of claim 24, wherein the oscillator is a distributed Braggreflector laser.
 27. The apparatus of claim 19, wherein the secondcircuit is a coherent detector.
 28. The apparatus of claim 27, whereinthe coherent detector includes a 90-degree optical hybrid circuitconfigured to receive the first portion of the light and the wavelengthdivision multiplexed optical signal.
 29. The apparatus of claim 19,wherein each of the plurality of optical signals is supplied by acorresponding one of a plurality of modulated sources.
 30. The apparatusof claim 29, wherein each of the plurality of modulated sources includesa laser and a modulator.
 31. The apparatus of claim 30, wherein thelaser is a distributed feedback laser.
 32. The apparatus of claim 30,wherein the laser is a distributed Bragg reflector laser.
 33. Theapparatus of claim 30, wherein the modulator includes a Mach-Zehndermodulator.
 34. The apparatus of claim 30, wherein the modulator includesan electro-absorption modulator.
 35. The apparatus of claim 30, whereineach of the plurality of signal channels includes an amplitude varyingelement.
 36. The apparatus of claim 35, wherein the plurality ofcharacteristics are selected from a group including an optical power ofthe light, a phase of the light, a carrier frequency of the modulatedsource, a carrier frequency of the laser, a wavelength of the modulatedsource, a wavelength of the laser, a gain of the amplitude varyingelement, an attenuation of the amplitude varying element, a bias of thelaser, and a bias of the modulator.
 37. The apparatus of claim 30,wherein each of the plurality of signal channels includes a thermaldevice configured to adjust a temperature of the laser, the temperatureof the laser being controlled by the thermal device in response to acorresponding one of the plurality of control signals.
 38. The apparatusof claim 37, wherein the thermal device of each of the plurality ofsignal channels is a heater.
 39. The apparatus of claim 37, wherein thethermal device of each of the plurality of signal channels is acorresponding one of a plurality of first thermal devices, the apparatusincluding a second thermal device configured to cooperate with theplurality of first thermal devices to adjust the temperature of laser ofeach of the plurality of signal channels.
 40. The apparatus of claim 39,wherein the second thermal device is a thermo-electric cooler.
 41. Theapparatus of claim 19, wherein the group of the plurality of frequenciesis a first group of the plurality of frequencies, the intensity peak ofthe filtered output is a first of a plurality of intensity peaks of thefiltered output, and the third of the plurality of frequencies is acorresponding one of a second group of the plurality of frequencies,each of the plurality of intensity peaks having a respective one of thesecond group of the plurality of frequencies.
 42. The apparatus of claim41, wherein each of the plurality of intensity peaks includes arespective one of a plurality of amplitudes.
 43. The apparatus of claim41, wherein the first and second of the plurality of frequencies definea first range of frequencies and the second group of the plurality offrequencies defines a second range of frequencies, the second range offrequencies being within the first range of frequencies.
 44. Theapparatus of claim 43, wherein the first group of the plurality offrequencies defines a third range of frequencies, the third range offrequencies being within the second range of frequencies.